Not true. Spy depends heavily on nonstock weapons to fit each situation, the kritz could be much better than an uber depending on the situation and the gunslinger can make a huge difference on offense or koth maps.
Just to name a few.
A few.
The setups I've found most useful are:
Scout-
Stock
Mad Milk or Bonk
Sandman
Soldier-
Stock
Reserve Shooter or Stock
Escape Plan
Pyro-
Degreaser
Flare Gun
Axtinguisher
Demo-
Stock
Stock
Stock or Ullapool
Heavy-
Stock
Sandvich
Stock
Engie-
Pomson
Wrangler
Jag or Gunslinger
Medic-
Blutsauger
Stock or Kritz, depending on situation
Ubersaw
Sniper-
Amateurs shouldn't play sniper.
But, when you get better, you can mess around with sniper.
Stock or Huntsman or Sydney Sleeper
Jarate (or Darwin's shield ONLY with sleeper when you have the set)
Bushwacka
Spy-
Same goes for spy.
Any watch, depends on which you like best.
DO NOT use the Eternal Reward at first, you WILL GET MAULED. Stock.
Stock or Enforcer. I use the Diamondback just to mess around.
The sapper depends on the situation. For nests with more than 1 engie, use the recorder. For just 1, use stock.
Hope you can use this!